Electrolytic production of mixed metal oxides



H. B. BEER 3,073,763

ELECTROLYTIC PRODUCTION OF MIXED METAL OXIDES Jan. 15, 1963 Filed April 13, 1959 United States Patent Ofilice 3,073,763 Patented Jan. 15, 1963 3,073,763 ELECTRQLYTIC PRODUCTION OF MIXED METAL OXIDES Henri Bernard Beer, The Hague, Netherlands, assignor to Magneto-Chemie N.V., Sehiedam, Netherlands, a corporation of the Netherlands Filed Apr. 13, 1959, Ser. No. 805,660 Claims priority, application Netherlands Apr. 17, 1958 6 (Ilaims. (Cl. 204-96) Patent application Ser. No. 700,560, -filed December 4, 1957, now abandoned, discloses a method of electrolytically preparing iron oxides or mixtures of metal oxides which predominantly-for more than 80 mol percentcontain iron oxide. According to said application the electrolysis is carried out by using an iron anode and, if desired, one or more other metal anodes, the cathode consisting of a conductive material e.g. a metal or carbon. As an electrolyte an alkali salt solution or a solution of a salt having a greater affinity to hydrogen than to iron is used. During the electrolysis the metals of the anodes will dissolve and the metal ions formed will come into contact with the alkaline medium which has formed adjacent the cathode. By taking care that there is an oxidizing atmosphere in the electrolysis bath eg by supplying oxygen or an oxidant thereto, the iron salt formed is converted into ferric oxide or ferrosoferric oxide which precipitates as an extremely fine, pure and uniform powder. In the presence of other metal ions formed from the relative metal anodes also these are converted into the oxides which coprecipitate together with the ferric oxide.

Because the composition of the electrolyte in the electrolysis bath does not substantially change during the precipitation described hereinbefore, the method can be carried out continuously and only a small amount of electrolyte is necessary for obtaining an unlimited amount of metal oxides. These can easily be purified from salts or other impurities by washing.

The electrolytical coprecipitation produces better results than the well-known chemical coprecipitation by means of an alkali lye because in the latter method initially the least soluble oxides will precipitate while for the precipitation of the better soluble oxides a sufficient amount of alkali is not always left. Furthermore the product is strongly contaminated by the equivalent amount of salt formed during the double conversion, which salt is to be carefully removed by washing. As a result said known method is not economical and besides it does not always result in a product having the desired properties. In the electrolytic method always so much alkali lye is formed as is necessary for the precipitation of the metals which have gone into solution at the anodes, while the original alkali salt always remains in solution and cannot contaminate the mixed oxide formed.

The invention relates to a method that is analogous to the method according to patent application Ser. No. 700,560, filed December 4, 1957, now abandoned, so to a method of electrolytically coprecipitating metal oxides, but in contradistinction to the method according to said patent application it is concerned with the coprecipitat-ion of metal oxides containing at most 80 mol percent of iron oxide.

The invention, therefore, relates to a method of coprecipitating metal oxides containing at most 80 mol percent of iron oxide, by reacting the metal ions in a predetermined proportion with an alkali solution, which method is characterized in that the coprecipitation is carried out by the electrolysis of an alkali salt solution while anodes of the metals the oxides of which are to be precipitated and/ or anodes of a conductive material which during the electrolysis do not disoslve but are surrounded by one or more oxides of the metals that are to be precipitated, are used, care being taken that the metals will dissolve in the desired proportion and will precipitate in the form of their oxides by the alkali formed during the electrolysis, before the metal ions reach the cathode. In order to ensure that the metals will dissolve in the ratio desired the electrolysis according to the invention is carried out with the help of anodes that are separated by insulating partitions, which do not extend entirely up to the cathode.

Whereas in the electrolytical preparation of mixed oxides containing more than mol percent of iron oxide we have found that when supplying oxygen or an oxidant the iron oxide is precipitated in a strongly active form and that also the other oxides readily precipitate, we have now found that in the presence of less iron or in the absence of iron the result desired is not always achieved by oxidation alone and that other measures are to be taken in order to prevent a metal precipitate from forming on the cathode.

Also in the method according to the invention it is necessary to effect so strong an oxidation at the cathode(s) as to prevent that the metal oxides in the vicinity of the cathode(s) are again reduced to the metals by the reducing atmosphere prevailing adjacent said cathode(s). To achieve this end it is possible, for example, to construct the cathode in the form of a tube and to perforate the tube below the level of the electrolyte. A strong flow of air, oxygen or air enriched with ozone can be blown through the openings in the tube or an oxidant, such as hydrogen peroxide may be dropwise introduced into the tube.

Furthermore the cathode(s) are such material that the formation of galvanic metal deposits thereon is rendered difficult. Examples of such materials are the metal having a resistant oxide film, which oxide film is not destroyed by the hydrogen liberated at the cathode(s). Said metals are for example aluminium, titanium and magnesium. The oxide film can also be formed by an additional treatment (by the anodization of aluminium and the like).

It is also suitable to so arrange the cathode(s) that a strongly alkaline medium will be formed about the cathode(s) so that the metal ions will be precipitated in the form of their oxides before they can reach the cathode(s). It is possible to wrap the cathode in a porous material which the alkaline catholyte will accumulate or to place the cathode in a porous vessel which is open at the lower side.

It is also possible of course to apply the above mentioned measures in combination.

The method according to the invention is generally carried out in a tank which itself is connected as cathode or in which one or more cathodes are arranged. As electrolyte an alkali salt or a mixture of alkali salts is used, the anions of which do not form insoluble salts with the metals of the anodes. The anodes which consist of the metals the oxides of which are to be coprecipi-tated are connected to separate ammeters so that it is possible to dose the amount of each metal that is to dissolve. partitions which do not cover the entire width of the electrolysis bath, but which 'all the same are so large as to prevent that the current supplied with different voltages to the anodes will run from the one anode to the others owing to the smaller mutual electric resistance instead of running to the cathode(s). However, said partitions must be so arranged relative to the cathode(s) that the anolytes can properly intermingle before the precipitation of the oxides takes place.

The intermixing of the anolytes can be promoted by means of a sluice between the anodes and the cathode(s), the anolytes being properly stirred before entering the preferably made of The anodes are separated by insulating I cathode zone. The sluice is suitably provided with one or more narrow slits in its centre.

As a means to understanding the concept of the invention, attention is now directed to the drawing. It will be noted, therefrom, as has been stated in the above, that the anodes 11, 11A and 118, comprising iron, nickel and zinc metals, respectively, are connected to separate ammeters 12, 12A, and 1213 so that the current energy level may be varied through separate variable resistances 13, 13A and 13B between the respective ammeters 12, 12A and 12B and the supply of current through line 19, which may be a battery 14, as shown. Additionally, it will be noted from the drawing that an oxidant area 15 is provided between the anodes and the cathode 16. The cathode is connected to the battery 14 by means of line 20. Furthermore, it will be noted that the bafiies 17A and 17B prevent direct contact between the anodes as was stated in the foregoing. The entire operation is contained in a vessel 18.

Instead of manufacturing the anodes from the metals that are to be coprecipitated as oxides it is also possible to start from the oxides themselves which are enclosed in a container in which centrally an electric conductor is arranged which does not dissolve in the anolyte, so that said anolyte is capable of dissolving the oxide. This renders it possible according to the invention to prepare a mixed oxide containing e.g. calcium oxide.

The method according to the invention can be carried out continuously and offers many advantages over the well-known chemical coprecipitation of oxides and over the method according to which it is attempted to convert the oxides into an intimate mixture by a protected grinding treatment and, if desired, by sintering.

The coprecipitates prepared according to the invention have a homogeneous structure and as a result they have in many cases other physical and also chemical properties than mixtures of the metal oxides that have only been mixed mechanically. It is even possible according to the invention to obtain mixed crystals.

The yield of the method according to the invention is excellent and generally amounts to more than 90% calculated on the amount of electric power supplied.

The mixed oxides prepared according to the invention may be applied in the paint industry, the electronic industry, the enamelling industry etc.

Example 1 The electrolysis is carried out in an insulated vessel containing as electrolyte an aqueous solution of 2% by weight of sodium nitrate. As anodes are used (1) a metal strip of lead having a length of 20 cm. and a width of cm., (2) a well premeable container filled with calcium oxide mixed with the electrolyte. In the centre of said container there is an insoluble electrode of e.g. platinum. By supplying electric current nitric acid will form at the anode whereby the calcium oxide is converted into calcium nitrate. The two anodes are connected with the source of direct current via separate control resistors and ammeters. By adjusting these to the desired current strength (lead-anode 3 A., platinum-anode 1 A.), the desired amount of lead and calcium ions will dissolve. The cathode consists of duraluminium. The salts formed by the electrolysis are mechanically thoroughly agitated in such a manner that they will mix substantially completely before the anolyte reaches the cathode and the lead and calcium are coprecipitated as a mixed oxide by the sodium hydroxide formed at the cathode. No metal will precipitate on the cathode. The coprecipitate of lead oxide and calcium oxide obtained (about 91% PbO and 9% CaO) after being washed and dried can be used as a pigment, which pigment has a substantially neutral reaction and which in paint-technical respects has much better properties than a mechanical mixture of finely divided oxides.

4 Example 2 The electrolysis is carried out in an insulated vessel containing as electrolyte an aqueous solution of 5% by weight of sodium sulphate. Rods of Zinc and copper (both having a length of 20 cm. and a width of 5 cm.) are used as anodes, said rods being separated from each other by a glassplate, which plate is placed at least 6 cm. from the cathode, so that the zinc and copper ions formed have an opportunity of intimately mixing with each other. The Zinc and the copper are each connected via a separate resistor with the source of direct current and the current stren th is so adjusted (zinc 18 A. copper 2 A.) that the required amounts of zinc and copper ions will dissolve. The cathode consists of stainless steel having a high silicon content. This cathode is arranged in a casing of hard asbestos which is open at the lower end. Air is blown into the catholyte which at the same time is mechanically agitated. The alkalinity of the electrolyte about the cathode remains so great that metal cannot deposit on the cathode. The coprecipitate of zinc oxide and copper oxide (about of ZnO and 1 of CuO) is washed and dried and appears to have excellent paint-technical properties as well as a strong anti-bacteriological action.

Example 3 The electrolysis is carried out in a tank of ebonite containing as electrolyte an aqueous solution of 5% by weight of potassium fluoride to which /r% by weight of potassium bifiuoride has been added (pH of the solution=5, 6). Tin and titanium serve as anodes both having a length of 20 cm. and a Width of 8 and 5 cm. respectively, and being separated in the manner indicated in Example 2 by a glass plate. The current strengths at the two anodes amount to 10 A. and 5 A. respectively. The cathode is a porous electrode consisting of a mixture of activated carbon and graphite. Said electrode is hollow, closed at the upper end and internally connected to an airpump which during the electrolysis continuously pumps fresh air through said porous carbon from above, which air left the bath via the electrolyte. In this manner there will continuously be a fresh layer of oxygen on the carbon, so that the reducing capacity of the hydrogen evolved is undone which prevents any metal from precipitating on the cathode.

In addition air or ozonized air is blown into the electrolyte through distributing pipes. The coprecipitated mixture of tin dioxide and titanium dioxide (about 80% S and 20% TiO after being washed and dried forms an excellent starting material for the manufacture of insulators, condensers, etc.

Example 4 The electrolysis is carried out in an insulated tank containing as electrolyte an aqueous solution of 3% by weight of sodium sulphate. The anodes are rods of Zinc, magnesium, manganese, cobalt and cadmium that are connected to a source of direct current as indicated in Example 2. All of the anodes have a length of 20 cm. and a width of 5 cm. The current strengths at these anodes amount to 1 A., 1.5 A., 15 A., 0.4 A. and 0.65 A. respectively. The anodes are again separated by glass plates for screening purposes while at right angles to said plates a porous plate is arranged which contains one or more narrow slits through which the anolyte can move towards the cathode Zone. However, before the anOlyte can move through said slits is vigorously agitated, so that an intimate mixing of the dissolved metal salts is effected. Owing to the provision of the correct slits in the porous plate which acts as a sluice between the anolyte and the catholyte a particularly good mixing is obtained, while owing to the slow flow of the anolyte the catholyte remains sufiiciently alkaline, so that no metal can deposit on the cathode which consists of copper. The coprecipitate of the oxides of Zinc, magnesium, manganese, cobalt and cadmium (about 3% ZnO, 2% MgO, 91.1% M1102,

0.9% C and 3% CdO) is washed and dried. When mixed with iron oxide, it forms an excellent raw material to be worked up into ferrites.

Example The electrolysis is carried out as in Example 2. The electrolyte consists of an aqueous solution of 5% by weight of sodium chloride. The anodes are rods (having a length of 20 cm. and a width of 5 cm.) of iron (1 A.) and manganese A.). Air is everywhere blown into the electrolyte through tubes both at the anodes, the cathodes (of iron) and the centre of the tank. After washing and drying the mixed oxide obtained (about 5% Fe O and 95% MnO forms an excellent raw material to be worked up as a depolariser in primary cells. As it is the iron oxide has a catalytic effect on the oxidation of the manganese oxide and as a result a cell having this raw material as a depolariser can produce current for an appreciably longer time. It. is tried to prepare this mixture by grinding the two oxides together, the mixing will never become so intimate that the iron oxide will not dissolve in the electrolyte of the primary cell and thus will attack the zinc cup, so that the keeping qualities greatly deteriorate.

Example 6 Sodium hypochlorite serves as an oxidant, which is formed in the electrolysis bath itself, for an aqueous solution of 3% by weight of sodium chloride is electrolysed, a platinum electrode being used as an auxiliary anode. The chlorine formed at said anode will react with the sodium hydroxide which is formed at the cathode which will result in the formation of sodium hypochlorite. By a vigorous mechanical agitation at good mixing is obtained. The anodes are bars (having a length of cm. and a width of 5 cm.) of manganese (1 A.) and nickel (6 A.). The auxiliary anode is a strip of platinum (having a length of 20 cm., a width of 2 cm., current strength 1.5 A.).

All of the anodes are separated from one another by glass sheets. The cathode is a plate of platinum. After washing and drying the mixed oxide of nickel and manganese obtained (about 84% NiO and 16% MnO forms an excellent raw material (so-called colouring oxides) for enamel.

I claim:

1. A method of electrolytically coprecipitating metal oxides comprising carrying out the coprecipitation by the electrolysis of an aqueous alkali metal salt solution, passing a current between at least two anodes and an inert depolarized cathode, said anodes selected from the group consisting of lead, zinc, copper, tin, titanium, magnesium, manganese, cobalt, cadmium, iron, nickel and calcium oxide surrounding a platinum anode, said anodes are separated by insulating partitions which do not entirely extend up to the cathode, controlling the electric energy supplied to the anodes so that the anodes will dissolve in the desired proportions and by the addition of an oxidizing gas to the bath will precipitate in the form of mixed oxides in the bath.

2. A method of electrolytically coprecipitating calcium oxide and lead oxide comprising carrying out the coprecipitation by the electrolysis of an aqueous alkali metal salt solution, passing a current between an anode of lead and an inert depolarized cathode, passing a current between an inert anode which is surrounded by calcium oxide and said inert depolarized cathode, said anodes are separated by insulating partitions which do not entirely extend up to the cathode, controlling the electric energy supplied to the anodes so that the lead and calcium oxide will dissolve in the desired proportions and by the addition of an oxidizing gas to the bath will precipitate in the form of mixed oxides in the bath.

3. A method of electrolytically coprecipitating zinc oxide and copper oxide comprising carrying out the coprecipitation by the electrolysis of an aqueous alkali metal salt solution, passing a current between an anode of zinc and an inert depolarized cathode, passing a current between an anode of copper and said inert depolarized cathode, said anodes are separated by insulating partitions which do not entirely extend up to the cathode, controlling the electric energy supplied to the anodes so that the zinc and copper will dissolve in the desired proportions and by the addition of an oxidizing gas to the bath will precipitate in the form of mixed oxides in the bath.

4. A method of electrolytically coprecipitating tin oxide and titanium oxide comprising carrying out the coprecipitation by the electrolysis of an aqueous alkali metal salt solution, passing a current between an anode of tin and an inert depolarized cathode, passing a current between an anode of titanium and said inert depolarized cathode, said anodes are separated by insulating partitions which do not entirely extend up to the cathode, controlling the electric energy supplied to the anodes so that the tin and titanium will dissolve in the desired proportions and by the addition of an oxidizing gas to. the bath will precipitate in the form of mixed oxides in the bath.

5. A method of electrolytically coprecipitating iron oxide and manganese oxide comprising carrying out the coprecipitation by the electrolysis of an aqueous alkali metal salt solution, passing a current between an anode of iron and an inert depolarized cathode, passing a current between an anode of manganese and said inert depolarized cathode, said anodes are separated by insulating partitions which do not entirely extend up to the cathode, controlling the electric energy supplied to the anodes so that the iron and manganese will dissolve in the desired proportions and by the addition of an oxidizing gas to the bath will precipitate in the form of mixed oxides in the bath.

6. A method of electrolytically coprecipitating manganese oxide and nickel oxide comprising carrying out the coprecipitation by the electrolysis of an aqueous alkali metal salt solution, passing a current between an anode of manganese and an inert depolarized cathode, passing a current between an anode of nickel and said inert depolarized cathode, said anodes are separated by insulating partitions which do not entirely extend up to the cathode, controlling the electric energy supplied to the anodes so that the manganese and nickel will dissolve in the desired proportions and by the addition of an oxidizing gas to the bath will precipitate in the form of mixed oxides in the bath.

References Cited in the file of this patent UNITED STATES PATENTS 554,718 McKenzie Feb. 18, 1896 626,547 Lucklow June 6, 1899 1,496,607 Saunders June 3, 1924 1,687,056 Carl Oct. 9, 1928 2,275,223 Hardoen Mar. 3, 1942 2,289,258 French July 7, 1942 2,425,058 Craig Aug. 5, 1947 2,778,787 Salt Jan. 22, 1957 2,796,357 Foss et al June 18,1957 2,880,123 Grulke et a1 Mar. 31, 1959 

1. A METHOD OF ELECTROLYTICALLY COPRECIPATING METAL OXIDES COMPRISING CARRYING OUT THE COPRECIPITATION BY THE ELECTROLYSIS OF AN AQUEOUS ALKALI METAL SALT SOLUTION, PASSING A CURRENT BETWEEN AT LEAST TWO ANODES AND AN INERT DEPOLARIZED CATHODE, SAID ANODES SELECTED FROM THE GROUP CONSISTING OF LEAD, ZINC, COPPER, TIN, TITANIUM, MAGNESIUM, MANGANESE, COBALT, CADMIUM, IRON, NICKEL AND CALCIUM OXIDE SURRONDING A PLATINUM ANODE, SAID ANODES ARE SEPARATED BY INSULATING PARTITIONS WHICH DO NOT ENTIRELY EXTEND UP TO THE CATHODE, CONTROLLING THE ELECTRIC ENERGY SUPPLIED TO THE ANODES SO THAT THE ANODES WILL DISSOLVE IN THE DESIRED PROPORTIONS AND BY THE ADDITION OF AN OXIDIZING GAS TO THE BATH WILL PRECIPITATE IN THE FORM OF MIXED OXIDES IN THE BATH. 